HOW ARE ANODES AND CATHODES OF ALUMINIUM ELECTROLYTIC CAPACITORS MADE?

Electrodes of aluminium electrolytic capacitors are made from high purity aluminium foils (purity 99.99% and above) treated to get maximum capacitance density on foil for desired application. The treatment involves etching and then forming oxide layer, and the process is different for anode and cathode foils. Thickness of foil may vary from 50 μ to 100 μ or more.

The figure explains basic structure of aluminium electrolytic capacitor. Cathode unavoidably has an extremely thin oxide layer, formed by natural reaction of oxide from air. Electrolyte is de facto cathode, while cathode foil serves to collect current from electrolyte to external leads. The paper is highly porous, allowing continuity and ion flow across the two sides.

Etching

Anode foil is first etched to increase effective surface area of foil to get maximum surface area. Thickness of anode foil may vary from 20 μ to 100 μ. Etching can increase the anode foil area by several times. Ratio of this increased area to area of plain foil is called ‘gain’, which can be as high as 100 for low voltage foils, and 20-25 times for high voltage foils.

Etching process involves running aluminium foil continuously through a chemical solution bath with a DC (sometimes AC) voltage or a combination of AC/DC voltage applied between bath and foil. This creates crests and valleys structure uniformly on the foil surface.  

The foil is kept totally free form any ionic impurities during process by washing with demineralized water multiple times before and after this chemical etching. Presence of even small impurities, particularly iron and chlorides, is harmful for foil and to capacitors thereafter.

Forming

The increased area after etching is available for creation of thin oxide dielectric layer by subsequent forming process. This involves creating an aluminium oxide (Al₂O₃) layer during a process called ‘formation’.  Etched foil is passed through a succession of baths with constant current applied between a forming electrolyte bath and foil. Voltage is increased gradually as oxide layer is formed. Final voltage may be from 140% to 200% of rated voltage of capacitor to be manufactured. Dielectric thickness depends on this final voltage and is of the order of 15 Å/Volt. Dielectric breakdown strength can be as high as 10⁶ V/mm.

Capacitance is then formed between aluminium anode foil base and electrolyte, the oxide layer serving as dielectric. Since capacitance is inversely proportional to dielectric thickness, oxide layer thickness is inversely proportional to formation voltage. However, in case of extremely fine etch structure, this relationship becomes non-linear.

Tab Foil

Capacitors have tabs taken out from anode and cathode foils for terminations for external connection and wiring. These are made using thick aluminium foils from 60 μ to 200 μ. Tabs have to be compatible with anode and cathode foils. For anode, tabs have to be etched and formed, since they also must withstand the working voltage between anode and electrolyte. Often, they are connected to anode by cold welding or punching process during or before capacitor winding process.

The damage caused to oxide layers of anode and tabs is replenished during forming process on overall capacitor after winding of capacitor. Tabs at cathode end (particularly for low voltage ratings) are often placed just in physical contact with cathode foil and held in place by winding pressure.

Tabs may be used in multiple numbers on foils at different places spread over foils in order to reduce ESR and increase current carrying capacity and also reduce its dielectric loss of capacitor. Thickness, width and length of tab varies widely, decided by design factors of capacitors, as also physical design considerations.

Cathode Foil

Cathode foil basically serves to collect current from electrolyte and pass it on to external circuit. It does not need oxide layer, but an extremely thin layer is nevertheless formed naturally due to action of atmospheric air. This creates a capacitance between cathode foil and electrolyte. In order to offset the effect of cathode capacitance, cathode foil is etched to increase its effective area up to 100 times.

Etched and formed anode and tab foils, as also cathode foils are made by specialized processes, and are supplied by few manufacturers around the world. Capacitor manufacturers have option from several types and specifications to choose from for their applications. Wide range of properties of cathode and anode foils are available to suit different applications and voltages.

Reformation of oxide layer during manufacture

After formation, the foils are slit into desired widths depending on size of capacitor to be made. Slitting process damages the oxide layer at the edges of foils. Further, handling of foil may also damage the oxide layer during manufacture. Welding or punching of tabs to anode foil causes damage to oxide layers at these places. A recovery process of applying a gradually increasing voltage on impregnated capacitor elements helps re-form the layer back.

Anode-electrolyte capacitance and cathode-electrolyte capacitance values are in series. High gain of cathode foil, couples with extreme thin oxide layer, makes the cathode capacitance several order higher than anode capacitance in overall electrolytic capacitor series combination. This makes cathode capacitance negligible compared to anode capacitance, and effectively the anode capacitance prevails.